The present invention relates to an improved gas dynamic laser incorporating a condensed explosive for producing a population inversion, for generating a high intensity pulse of coherent radiation. A military application of such a laser results from the ability to concentrate large quantities of energy practically instantaneously onto a small region at a considerable distance from the source. A laser includes a lasing medium, which may be either solid, liquid or gas, in which electron or molecular energy is stored in a particular way to produce a population inversion. This means that a high or upper energy (or excited) state or level of the medium is densely populated (i.e. many of the molecules are in the high energy state) whereas a normally well-populated lower energy state or level is relatively empty. If a population inversion exists, the energy difference between the high and low energy states can be released almost instantaneously as a coherent beam or pulse of radiation with a defined wavelength and phase. A beam or pulse can in principle be maintained for a significant duration by continually replenishing the upper energy level and depleting the lower energy level. The storage of energy in the lasing medium is sometimes called "pumping".
A gas dynamic laser is one in which the lasing medium is a gas, or a mixture of gases, such as carbon dioxide (CO.sub.2) and nitrogen (N.sub.2). Such gas lasers usually include a source of the gas for the lasing medium and a separate means for pumping the medium to produce the necessary population inversion. In Reuter, et al., U.S. Pat. No. 3,262,071, the gaseous lasing medium is pumped by radiation generated by the explosion of a carbon rod. DeMent U.S. Pat. No. 3,414,868 discloses a laser in which a solid, liquid or gas lasing medium is pumped by light generated by an exploding wire or chemical explosive such as TNT, PETN, TNM, RDX, etc., or by burning metal particles such as aluminum. Gregg U.S. Pat. No. 3,623,145 discloses a laser in which the lasing medium is an explosive gaseous mixture of hydrogen and a nitrogen-fluorine compound, such as N.sub.2 F.sub.2, which is ignited by flash photolysis or an electron beam, and the resulting chemical explosion pumps a lasing chemical species, hydrogen, deuterium or tritium fluoride, which is formed in the chemical explosion.
An abstract by C. P. Robinson and J. A. Sullivan, of Los Alamos Scientific Laboratory, in the Bulletin of the American Physical Society, Februrary, 1972, p. 67, briefly described some results obtained with a gas laser in which the laser gases were chemically produced in the explosion of a condensed explosive, HNB (hexanitrosobenzene-C.sub.6 N.sub.6 O.sub.6), and the gases were thermally pumped at high stagnation temperatures (3 to 5 .times. 10.sup.3 .degree. K.) over a wide range of pressures. A gain of 3% per cm. was reported in the abstract. The condensed explosive gas laser of Robinson and Sullivan is the subject matter of Robinson et al. U.S. Pat. No. 3,904,985, issued Sept. 9, 1975. In a private communication, we have been informed that Robinson and Sullivan also experimented with PETN (pentaerythritol tetranitrate-C.sub.5 H.sub.8 N.sub.4 O.sub.12), which did not form a good lasing mixture (which they attributed to the excess of water vapor, about 4.5%, present) and also considered the use of pentanitroaniline (C.sub.6 H.sub.2 N.sub.6 O.sub.10), alone, and mixtures of TNM (tetranitromethane-CN.sub.4 O.sub.8) plus acrylonitrile (C.sub.3 NH.sub.3) and TNM plus HNB, as the condensed explosive.
Calculations made by us showed that when HNB alone (one explosive used by Robinson and Sullivan) is exploded and the explosion products are allowed to expand, both the pressure and the temperature in the products decrease along the expansion path as shown in Table I:
Table I ______________________________________ Pressure Temperature ______________________________________ (Atmospheres) (.degree. K) 1093 1412 497 1298 226 1204 103 1123 47 1051 21 985 ______________________________________
A corresponding table for PETN (the other explosive tried by Robinson and Sullivan) is shown in Table II:
Table II ______________________________________ Pressure Temperature ______________________________________ (Atmospheres) (.degree. K) 5817 1277 2620 1121 1180 991 532 877 ______________________________________
It can be seen from these tables that with either HNB or PETN, alone, it is necessary to operate (provide lasing) in a very high pressure region (at least 60 atmospheres in the case of HNB) in order to have a temperature of at least 1100.degree. K. This is true of all other organic explosives, such as TNM. Another disadvantage of using any of HNB, TNM or PETN, alone, as the condensed explosive in a laser is that, at reasonably low pressures and reasonably high temperatures, the calculated values of the relaxation time .tau..sub.II of the upper lasing level, the characteristic expansion time .tau..sub.E of the lasing medium, and the relaxation time .tau..sub.I of the lower lasing level do not satisfy the relationship EQU .tau..sub.II &gt;.tau..sub.E &gt;.tau..sub.I,
required to produce a population inversion in the expansion region beyond the nozzle throat. Instead, .tau..sub.II is always less than, instead of greater than, .tau..sub.E.
In accordance with the present invention, the disadvantages pointed out above are avoided by using, as the condensed explosive of a gas dynamic laser, a condensed mixture of one or more nonhydrogenous organic explosive compounds, such as TNM, with a sufficient amount of aluminum or zirconium metal powder to supply energy to the products so that a temperature of at least 1100.degree. K. can be achieved at the nozzle throat at a pressure of not more than 10 atmospheres, and simultaneously adjusting the proportions (percentages) of the mixture to produce conditions satisfying the above inequality. The aluminum powder adds various aluminum compounds, including aluminum oxide (Al.sub.2 O.sub.3), to the explosion products. The beam attenuation produced in the lasing medium by the solid Al.sub.2 O.sub.3 is insufficient to prevent lasing, because of the relatively small quantity present and the extremely small size and optical surface properties of the particles.
Other compounds that may be used instead of TNM include dinitrobfurazanyl (C.sub.4 N.sub.6 O.sub.6) and trinitro, triazido-benzene (C.sub.6 N.sub.12 O.sub.6). The choice is based on compatibility, appropriateness for casting or pressing, sensitivity, storability, degradation and other properties related to qualificatons as a practical explosive. For this application, the additional required feature is the correct elemental composition of the total mixture for adequate energy release and classification as a detonating explosive.
Each of the explosive compounds named above except PETN is nonhydrogenous. Calculations have shown that water vapor in some explosion products (as with PETN) tends to inhibit or quench the formation of the population inversion essential to lasing.
As an example of an apparatus for carrying out the method of the present invention, the confining chamber of the laser may be a double-walled metal structure made up of two concentric elongated metal members with part of the space therebetween filled with a resilient cushioning material, and the explosive mixture may be in the form of a solid element disposed centrally within the inner member near one end. A convergent-divergent nozzle plate within the inner member, comprising at least one throat spaced from the explosive element, defines the input end of an expansion region. The generated laser output beam, or pulse, may be transmitted through optical windows on the double wall of the chamber structure, opposite the expansion region.